1. Field of the Invention
The present invention relates to a data carrier system for signal transmission between a main apparatus and a tag, and more particularly to a data carrier system for signal transmission between a main apparatus equipped with a power source and a tag with no power source, by utilizing electromagnetic coupling between a transmission/reception coil of the main apparatus and a transmission/reception coil of the tag.
2. Description of the Related Art
A data carrier system for signal transmission between a main apparatus and a tag by radio waves is used, for example, with a gate system for a ski lift. The tag has a transmission/reception mechanism and a memory and is held by a skier on his or her arm like a wrist watch. The memory stores various data, for example, a term for using the lift, the number of remaining times allowed to use the lift, and the like. The main apparatus issues an enquiry to the tag. As a skier passes near the main apparatus, the tag receives a signal from the main apparatus and starts its operation while producing electric power from the received signal. The tag reads data from its memory, according to the contents of the enquiry, and sends it back to the main apparatus. In accordance with this data transmitted from the tag, the main apparatus judges whether the skier is allowed to use the lift, and controls the gate to open or close. Such a data carrier system is widely used in various applications, not only in a ski lift gate system but also in an automatic train gate system, in an antitheft system in which a tag is fixed to each good for sale, and in other systems.
In such a data carrier system, a communication distance between the main apparatus and the tag is generally set to about 20 cm in order to discriminate individually the different tags passing near the main apparatus. However, this communication distance may be required to be set longer, depending upon the use of the data carrier system. In this case, it is required to produce a strong signal from the main apparatus. However, since the tag operates by an electric power produced from its receiving signal, it is necessary for the main apparatus produce a fairly strong output. Thus, the main apparatus transmits a strong signal even when the tag passes very near the main apparatus, resulting in wasteful consumption of the power in the main apparatus.
One example of such systems is disclosed, for example, in U.S. Pat. No. 5,500,651, entitled "System and Method for Reading Multiple RF-ID transponders" issued to J. Schuermann and corresponding Japanese Patent Kokai JP-A-8-180152. In this system, signals are transmitted between transponders and a reader. However, an object of this patent is to enable the reader to discriminate many transponders, which pass near the reader, from one another. However, the patent does not describe how to increase a communication service distance between the reader and each transponder.